Metallic powders



Filed May 18, 1951 C03/AMJ@ S04 f- 'f rj-g i wam Patented Jan. 12A, 1954 UNITED vS 'i'A'i'j 'S 'BET el i@ METALLIC POWDERS Charles Marquaire, La Tronche, France, assigner to Societe .dElectro-Chiinie dElcctro-Metallurgie et des .Acieries Electriques dUgine, Paris, France, a corporation of France A great number of methods are known for the manufacture of metallic powders and especially of iron powders. Thus, it is known to manufacture said powders by grinding, by electrolysis, by atomization of liquid metals, by volatilization of metals, by reduction of oxides or metallic compounds or other similar methods.

As the applications of the powders grow vconsiderably in industrial development, the specific properties of said powders must be more and more defined as a function of the uses which are desired for the same.

It is known to manufacture metallic powders for the preparation of permanent magnets with very high magnetic characteristics, more particularly by a method which consists in making permanent magnets by agglomeration, by way of simple compression of powder obtained by reduction under suitable conditions of metallic formates, oXalates, hydroxides and carbonates. Such powders show the remarkable property of possessing an exceedingly high coercive force Awhich is maintained after the compression and owing to which the obtained magnets show very high magnetic characteristics.

A method has also been developed for the preparation from formates of an iron powder making it possible by a simple agglomeration to manufacture compact and coherent pieces. The iron powder obtained by this method possesses no characteristics which make it suitable for the manufacture of permanent magnets but, on the other hand, it is particularly well suited for the manufacture of iron articles by simple compression. However, this method is comparatively expensive owing to the high price of formic acid. Furthermore, the obtained powder always shows an appreciable carbon content.

The present invention has for its object to provide for the manufacture of metallic powders and especially iron powders which are particularly pure and suited for the manufacture of metallic articles by a simple compression inthe cold without sintering or with a sintering effected at a lower temperature and more rapidly than in the case of the hitherto used powders.

The powders obtained by the method in accordance with the present invention show a very small coercive force which excludes their use for the manufacture of permanent magnets but makes them, on the other hand, particularly well suited for the manufacture of articles which are to shew a high magnetic permeability.

The methodV in accordance with the invention resorts to the decomposition and to the reduction of the carbonates but this decomposition and this reduction are performed, as indicated hereafter, under particular conditions which are such that the obtained nal product is endowed with properties which distinguish itA from the prior products resulting from the decomposition of carbonates under other conditions and, more particularly, as stated more precisely hereinabove, it practically possesses no coercive force.

This methodl constitutes, furthermore., a new and particularly economical method for the obtention of powders. It essentially comprises a series of chemical reactions which consist in attacking the metal or metals which it is desired to obtain in thev state of powders with an acidic solution causing an escape of hydrogen and bringing forth the formation of a solution of the corresponding salt, in forming the carbonate of this metal or metals by the action of ammonium carbonate on said solution with formation of a solution of thev ammonium salt of the acid which is used, in decomposing said carbonates into oxides and carbonio gas, in reducing the so formed oxides by hydrogen at a temperature higher than 506 C. whereafter one preferably proceeds to the regeneration of the ammonium carbonate by the action of lime on the ammonium salt which gives rise to formation of ammonia on which carbonio gas and water are caused to act.

In order that the invention may be better understood the series of the reactions which occur in the particularly interesting case of the production of an iron powder from scrap iron and sulphuric acid is given hereunder by way of example:

(l.) They iron is attacked in the form, for eX- ample, of rather line divided scraps by a diluted solution. of sulphuric acid. The hydrogen which escapes in the course of the operation is collected in a gas-holder, possibly after purification, and kept with a view to using itin a later step of the method whichv will be disclosed hereafter.

The attack is carried. out in the presence of an excessof ironinorder to use up almost totally the acid which is employed. The reaction which is exothermic requires onlyl a small supply of heat. The initial concentration of the acid is chosen insuch a mannerthat, after the'reaction, a solution of. ferreussulphateinthe neighborhood of the saturation and a concentration of free residual acid of about 1% are obtained. For eX- ample, one may start with a 25% solution of sul- `ima-rre aci-d end with a slightly acid ferrous sulphate soluticnv saturatedto 60%.

(21)' The ferrous sulphate solution is ltered and mixed under shelter from air with an ammonium carbonate solution saturated with carbonic gas by bubbling of said gas at atmospheric pressure and at a temperature round 50 C. The reaction for the formation of the iron carbonate which is aided by the stirring produced by the bubbling is immediate and the iron carbonate precipitates. The ammonium sulphate solution resulting from reaction is removed and the carbonate precipitate is washed preferably with hot water saturated with carbonio gas. This precipitate is then again dried under shelter from air. A particularly convenient means for performing this drying consists in atomizing in an atmosphere of hot carbonio gas the sludge formed by said precipitate.

(3) One proceeds to the decomposition of the so obtained iron carbonate by heat which gives rise to the formation of ferrous oxide and to an escape of carbonio gas. This decomposition is preferably performed at a lowtemperature, for example in the neighbourhood of 350 C. by removing as rapidly as possible from the reaction field the carbonio gas which settles as it appears either through a fall of the pressure or through carrying forth. bv means of a neutral or reducing gas. It is recommended to collect the carbonio gas which escapes in the course of said decomposition in order to use it in a later step of the method.

4) The reduction of the ferrous oxide hy hydrogen is then performed at a temperature of more than 500 f?. by usine. for eramole, the hydrogen produced by the initial attack of the iron by sulnhuric acid, Said attack yields the necessarv ouantitv of hydrogen except for losses.

The reduction temperature and the hydrogen output must he chosen while observing the following.r considerations according to the properties which are to be conferred to the powder:

(al The apparent density of the powder is the smaller the lower the reduction temperature. However, this latter must never fall below 500 C'. since otherwise one would obtain a comparatively impure nvrophoric powder endowed with a high coercive force.

(bl If the reduction temperature increases the purity of the iron increases whilst its neness decreases as well as its grinding facility.

(c) For a predetermined reduction temoerature a powder is obtained which is the purer, in a given time, the higher the hydrogen output without substantially changing its apparent density. En any case it is of interest to work with previously purified and very well dried hydro'ren.

A high output and a high drying of the hydrogen make is possible to reduce the duration of the treatment or to reduce the temperature of said treatment.

It is possible to carry out the method bv directly reducing the carbonate by hydrogen while takina cave to provide a temperature level at about S50-450 C. under a very small hydrogen output during a predetermined period of time which is sufficient in order to obtain the total decomposition of the iron carbonate. After this level one establishes the normal working temperature and the normal hydrogen output for the reduction, This modication of the method of manufacture makes the recoverv of the CO9. a. little more diicult but it simplifies the handling of the powder.

On the other hand. it is desirable to apply the known method of reduction in two steps while `proceeding to a first at abouiatmefifi C., in the course of which the iron is almost totally reduced and which leads to the obtention of a line very friable powder which is crushed, screened and then submitted to a second reduction at a temperature chosen according to the desired oxygen content and neness.

This method makes it possible to obtain a iine non-sintered powder at the outlet of the reduction oven.

(5) It is possible to treat with lime the ammonium sulphate solution obtained in the second step of the method which gives rise to an escape of ammonia which is collected in solution in water. y

(6) The carbonio gas produced in the third step of the method is caused to pass into this latter solution in order to obtain the ammonium carbonate solution saturated with carbonio gas which is necessary in the second step.

It is to be noted that in the steps 5 and 6 the recovered ammonia correspond except for losses to the quantity which is necessary for the pre cipitation of the iron carbonate in the second step of the method.

VIhe accompanying drawing shows the whole of the reactions which are used in the above described method while taking into account the possible recoveries and which may be summed up in the following manner:

This series of operations could be performed under quite similar conditions with hydrochloric acid instead of sulphuric acid. The method would be less economical but the obtained pcwder would be purer and, more particularly, entirely free from sulphur without, however, containing chlorine, owing to the facility of removal of the ammonium chloride by Washing and to its volatility in the reduction step.

In a general way, other acids and other metals than iron may also be used, provided the action of the acid on the metal iiberates hydrogen with formation of a soluble salt which may be con- Verted into a carbonate by the action of ammonium carbonate.

When applied to the preparation of iron powder the method in accordance with the invention conduces to the obtention of a powder the main properties of which are: a very high purity, a very small apparent density, a remarkable ntness for agglomeration by a simple compression in the cold and a high iineness.

It is known that in the preparation of the powders in a chemical way the manner in which the starting compound is prepared exerts a considerable influence on the properties of the obtained powders. More precisely, in the manufacturing method in accordance with the inven- .tion it is the. preparation of the carbonate as it I. Purity The obtained powder is practically free from sulphur, phosphorus, silicon and carbon. On the contrary, the manganese content is equal to that it is possible to obtain, Whiie working with hydro ehioijic acid, a powder the impurity contents of which. are vthe*foiilowinggr Mode introduction Body A Single 2reductions, ,reslnotionet gsttqg 0. 2'7` 0; 27 0. .005 {;7005 002 0, .002 o. 02v 0. 02 0-008 0.D05 0,3.5 (LA The iron powder- Iobtained,by deeomposito of formate is much pure: :et

O2 and 0.031% of C; Its Sulh f, silicon contents are .ceeriy biglie; 't the iron obtainedv in4 accordano@ with the. 1o invention precisely oss-ingV to .the` Diepe# i? the iron formate through crystal-Illmatic If the pmi/siemA obtained in accerdaneewith.- the invention is compared with two :iron PQWQS which are on the market ein@ used imL it Yeiy large scale, viz. the powderreduced from spongeiron and the powder obtained by atomzdftign. .it will be found' that, onthe one handttlieipqiil-.QT reduced from spongeeiron is.I much lesispiire Owing the ore, its siiicon content being, more partici;- iarl-y, very high and, on the. 4other hifi-nii,r the. iron powder obtained by atomization sho-wsthe same impurities as the startingmetatthis method tieing such that itA does not purity/the powder.

The following table indicates ley` wat* O 9.31.- parison the current impurity` corriente of' both said powders in the commercial state-z o Y o Mn s -P sie,

Ironsponge 0.05 1.7 0.00 0.005 0.01 1.5 Atgrgizeeiron 0.1 0.8; 02s 0,04 0,09 0.20

1I. Geometrzcal properties` of the powder After reduction oneA generally: obteLns; for rednetion, temperatures which. sic.- nst eXoeee 899 C. a powder eeeloniereted inte e sponge wireli.A is very posons (apparent density about QJ?) which may easily be crushed to powder with a device such as, for example, a cyii-nder grinding machine.

reduction te-nruceeatin-es are mmh lower than those which are necessary' for time. einer."

methods.. 'this Lowering is due te the condition of the iron which app-eers .the tenons state owing; te the deeornsesitien of the iron carbonate 'dieinee inosphere..

W osof; this powder are: the nneness. ci thief sittin,l the very irnegiileu shape and; the porous structure of the sra-in es. well as tine veresmalieppsteet density of the powder.

'From carbonate with powders in accordance with the invention arey .characterized by@ small apparent density and a large surface of the particles which are the causes o f the. streng-th of tiney agglomeraties end of the sintering facility of the powders .oi which .they 347.6- fQlmQd.

III. Compresszbz'lity and properties of the agglomexcites connessioni-ty ci the powder obtained .in etordanee 'th t tion 1s veryA good for` a clp-:(51.7 13:14 legst/mm.2 AB=120 @Powder ted-used .et .e higher temperature the ehefieeteristies are e 4little poorer; the density is higher, but the strength is .smaller for equal density Qi? even. ier o eqiiel compression.-

edventeseons, .mechanical qualities of the eselomeretes of; owders obtained in accordano-e with the invention 'make it possible to use these powders ier .making pole pieces or magnetic oirhye simple eornpression in the .cold without subsequent, sinteiing.v The .magnetic characteristios are very acceptable for .certain uses for .eigenipleinmaenetie circuits with. a flux of e. oonstantdireotion,

1 n e eloinerete obtained by Compression sie powder rediioedet D. and ity of 6.4 has a tensile strength of 12 kgs/mm?, a Brineli hardness of 115, a coercive iieid ci.A 15. oerstsds a.. sufficient permeability for clot n an induction oi 10,900. geuss in e .ieidioi .1.50 ersteds-` The nermeebinty is meiioreted leyY usine powder reduced et a higher temioerstirre .by egeioineretine under a higher Piessiiife..

All these preceding properties cannot be found in any other known powder.

When considering only the fine powders, for the others are much poorer, as regards the strength of the agglomerato, it may be seen that: The powders reduced from technical oxide or sponge-iron yield agglomerates having a much lower strength: for example, the exceptionally pure and line sponge-iron powder 50p.) when compressed under 8 T/cm.2 gives an agglomerate having magnetic characteristics which are comparable with those of the powder obtained in accordance with the invention but the strength of which is lower (S legs/mm.2 for a density of 6.8).

The same is true for the iron powder obtained by decomposition of carbonyl iron which in the compressed state shows magnetic properties which are a little poorer and above all a poor strength (R=4 kga/mm.2 for a density of 6.6 obtained under 8 'TF/cm2) The iron powder obtained from formate gives agglomerates having a good strength but owing to the fact that its purity is not so high the magnetic characteristics are lower.

It is to be noted, furthermore, that the powders obtained in accordance with the invention may be compressed in the presence of a binding agent in order to give the agglomerate certain particular properties such as, for example, a higher eleotric resistivity.

IV. Sintering of the aggiomerates and properties of the sintered pieces The essential characteristic of the powders obtained in accordance with the invention is their remarkable tness for sintering at a low temperature owing to theii` purity, to their neness and to the very activated structure of the'large surface of their particles.

These conditions render possible a sintering at a comparatively low temperature which in any oase is lower than the conversion point (an) of pure iron. For sintering temperatures below 1200 C., the maximum density and hardness are obtained at a sintering temperature of 850-875 C.

Thus, a powder such as the previously described one when reduced at 609 C., containing 0.4% of oxygen and compressed under 7 T/cm.2 gives an agglomerate having a density of 6.3; when Sintered for one hour at 850 C. in hydrogen this agglomerate gives a sintered piece having a density of 7.2 and a tensile strength o 25 kgs/mm.2 and a Brlnell hardness of 99 whereas under the same conditions no known iron powder gives a higher strength than 18 kgs/mm?.

The sintering of an agglomerate which is identical with the preceding one at a higher temperature (lodo-1190" C.) gives a piece the strength of which is slightly lower.

Among the other known iron powders only the Very fine powders formed from pure oxides can give comparable results but only for the characteristics of sintered pieces.

The crushed, atomized or electrolytic current commercial powders yield the values of the preceding characteristics only after a sintering at a higher temperature, which is higher than 1200? C.

V. Price On the base of a plant capable of producing from to 20 tons of powder per month the cost of an iron powder obtained in accordance with the invention is comp-arable with that of the coarse iron powders (sponge-iron, atomized or ground powders) having a poor purity and is 2 to Number oxide to iron.

172. Edited by Goetzel. l Interscience Publishers, New York.

5 times less than that of the other commercial iron powders having a comparable purity.

What I claim is:

l. A method for the manufacture of metallic powders in a high state of purity well tted for making metallic articles by cold compression and capable of being sintered rapidly at a comparatively low temperature, comprising the steps of dissolving in an -acid at least one metal which it is desired to obtain in powder condition, separating the solution of metal salt from the residue, mixing the solution of metal salt with a solution of ammonium carbonate while bubbling carbon dioxide through the mixture to saturate it and precipitate carbonate or the metal and form a solution of an ammonium salt of the acid, separating the precipitated carbonate of the metal, heating the precipitated carbonate of the vmetal while sheltered from air to decompose it into oxide of the metal and carbonio gas, and heating the oxide of the metal in a reducing atmosphere at a temperature of at least 500 C. to reduce the oxide of the metal to metal.

2. A method according to claim l, wherein the reducing latmosphere is an atmosphere of hydrogen.

3. A method for the manufacture of iron powders in a high state of purity well fitted for making articles by cold compression and capable of being sintered rapidly'ata Vcomparatively low temperature, comprising the 4steps of dissolving iron in an acid, separating the solution of iron salt from the residue, mixing the solution of iron salt with a solution of ammonium carbonate While bubbling carbon dioxide through the mixture to saturate it and precipitate iron carbonate and form `a solution of an ammonium salt of the acid, separating the precipitated iron carbonate,

heating the precipitated iron carbonate while sheltered from air to decompose it into iron oxide and carbonio gas, and heating the iron oxide in a reducing atmosphere at a temperature of at least l500" C. to reduce the iron oxide to iron.

4. A method for the manufacture of iron powders in a state of high purity well tted for making varticles by cold compression and cap-able of being sintered rapidly at a comparatively low temperature, comprising the steps or" dissolving iron in sulphuric acid, separating the solution of 'iron'sulphate from the residue, mixing the solution of iron sulphate with a solution of ammonimonium sulphate, separating the precipitated iron carbonate, heating the precipitated iron car- `bonate while sheltered from air to decompose it into iron oxide and carbonio gas, and heating the iron oxide in an atmosphere of hydrogen at a temperature of at least 500 C. to reduce the iron CHARLES MARQUAIRE.

A References Cited in the leof this patent UNITED STATES PA'TENIS Name Date 802,928 Fireman Oct. 24, 1905 A 997,237 Carrick July 4, 1911 2,254,976 Powell Sept. 2, 1941 2,441,770 Kroll May 18, 1948 OTHER REFERENCES Treatise on Powder Metallurgy, vol. I, page Published in 1949 by 

1. A METHOD FOR THE MANUFACTURE OF METALLIC POWDERS IN A HIGH STATE OF PURITY WELL FITTED FOR MAKING METALLIC ARTICLES BY COLD COMPRESSION AND CAPABLE OF BEING SINTERED RAPIDLY AT A COMPARATIVELY LOW TEMPERATURE, COMPRISING THE STEPS OF DISSOLVING IN AN ACID AT LEAST ONE METAL WHICH IT IS DESIRED TO OBTAIN IN POWDER CONDITION, SEPARATING THE SOLUTION OF METAL SALT FROM THE RESIDUE, MIXING THE SOLUTION OF METAL SALT WITH A SOLUTION OF AMMONIUM CARBONATE WHILE BUBBLING CARBON DIOXIDE THROUGH THE MIXTURE TO SATURATE IT AND PRECIPITATE CARBONATE OF THE METAL AND FORM A SOLUTION OF AN AMMONIUM SALT OF THE ACID, SEPARATING THE PRECIPITATED CARBONATE OF THE METAL, HEATING THE PRECIPITATED CARBONATE OF THE METAL WHILE SHELTERED FROM AIR TO DECOMPOSE IT INTO OXIDE OF THE METAL AND CARBONIC GAS, AND HEATING THE OXIDES OF THE METAL IN A REDUCING ATMOSPHERE AT A TEMPERATURE OF AT LEAST 500* C. TO REDUCE THE OXIDE OF THE METAL TO METAL. 